Sea Urchins
Ben Hirsh
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([1] ) (BS)

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Classification/Diagnostic Characteristics

Sea Urchin Anatomy (3) (SJ)

  • Kingdom: Animalia
Phylum: Echinodermata
Class: Echinoidea
  • Sea Urchins are deuterostomes which is a phyla of animal in which during early embryonic development, the mouth forms on the opposite position from the blastopore, and the blastopore develops into the anus.
  • Under the classification of deuterostome, the sea urchin is considered an echinoderm which also includes sea stars and other close relatives.
  • Sea Urchins are not vertebrates meaning that they do not have a vertebral column as their primary support structure.
  • All echinoderms have a unique symmetry along five radial axes known as pentaradial symmetry, but have bilaterally symmetrical larva.
  • Sea Urchins do not have heads.

The Echinus melo or water melon sea urchin
The Echinus melo or water melon sea urchin

Relationship to Humans

  • Both sea Urchins and Humans are deuterostomes as defined above.
  • Both Sea Urchins and Humans have internal skeletal systems.
  • People throughout the world eat the roe (eggs) of sea urchin. Fisheries used to use red sea urchin as the main source of this roe, but they were over fished and preyed on by sea otters, causing them to decline sharply in numbers. Now, fisheries have tried to start fishing the purple sea urchins, but because they yield less roe, a large industry has yet to develop. (6)(WSS)
  • Sea Urchins primary diet consists of algae and many sea urchins specifically prefer a certain type of giant kelp that often form "forests" in the ocean. Sea urchins have been known to wipe out massive amounts of these kelp fields. The kelp is harvested for a specific chemical called algin which is used in the manufacture of plastics. A greater push to fish other types of sea urchin has been made because of this problem. (13) (DA)
  • The sea urchin roe, often used in Japanese sushi as well as Korean and Chilean cuisine, is called "uni". After gaining knowledge about through the food industry, we have learned that the "uni" is not roe (the eggs) at all, rather it is the organs that produce the roe: the gonads. Generally, five strips of the "uni" reside within the sea urchin and are a yellowish orange custard like substance. After collection of the sea urchins, they are stripped of the uni and then the uni is packaged before being exported. (15) (PS)

Habitat and Niche

  • Sea Urchins are important grazers of algae in the world’s oceans.
  • Most Sea Urchins live in water environments.
  • Sea urchins can live in both cold or warm ocean water.
  • They also can live in rock pools, mud, wave- exposed rocks, coral reefs in kelp forrests, and sea grass beds.
  • They live in places where they have sources of algae, sea grass, and seaweeds to eat.
  • Sea urchins lodge themselves halfway into the sand, mud, or hole in order not to be swept away in large waves and currents.
  • They are nocturnal, hiding in holes and crevasses during the day, and feeding at night [2] (SM)
  • they live in warm and cold water (HSC)

Predator Avoidance

  • Are unable to move quickly if threatened by a predator.
  • Sea Urchins have a unique defense mechanism of sharp spikes surrounding their entire spherical body to protect itself from predators.
  • Certain species of sea urchins have jaws called called pedicellariae (singular pedicellaria) dispersed between the spines and tube feet of the sea urchin. While three types of pedicellariae are continuously biting and are used for crunching larvae and spores trying to attach to the sea urchin, the last type, named globiferous pedicellariae, helps protect the sea urchin from predators. Globiferous pedicellariae have three sharp tips with poison ducts leading up to them. If a predator such as specific species of sea stars approaches, the sea urchin will flatten its spikes, exposing its globerifous pedicellariae. If the sea star gets close to the sea urchin's globiferous pedicellariae, they will respond to chemicals released from the sea star's flesh, clamping onto the sea star and releasing toxin. The sea star will feel discomfort, and in some cases, release the sea urchin. [3] (FZ)

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Ability of sharp spines (CC)

Nutrient Acquisition

  • Sea Urchins have an oral side which contains a mouth where nutrients can enter the body.
  • The Sea Urchins have a water vascular system consisting of water filled channels that lead to tube feet which play a role in feeding.
  • Using a complex rasping structure Sea Urchins are able to scrape algae from rocks to eat.
  • They also obtain algae from plankton using their tube feet to catch and bring them into their body.

Reproduction and life cycle

  • Sea Urchin ova release species-specific chemicals that attract Sea Urchin sperm.
  • Sperm must pass through the jelly coat and vitelline envelope before coming to the ovum’s plasma membrane.
  • When the sperm reaches the jelly coat it’s acrosome (head) releases enzymes and other proteins which breakdown a hole through the jelly coat exposing the vitelline envelope. This is called the acrosomal reaction.
  • Next, the acrosomal process, an extension made from actin extends out of the sperm head. It is coated in species-specific molecules called bindins.
  • The bindins bind to receptor on the vitelline envelope and on the egg membrane. This triggers the fast block to polyspermy by changing the electric charge on the membrane.
  • The sperm and ovum membranes fuse and the sperm releases its centriole and nucleus into the ovum.
  • Enzymes remove sperm-binding receptors as a part of the slow block polyspermy.
  • Sea Urchins spawn which means they directly release their sperm into the open water and bindins helps locate and attach the sperm to a Sea Urchin ovum.
  • If a piece of a Sea Urchin is cut off it can grow into a new Sea Urchin. this phenomenon is known as regeneration.

Growth and Development

  • Sea Urchin larva is bilaterally symmetrical, but as it matures the Sea Urchin becomes pentaradially symmetrical.
  • Are organized with an oral side and an aboral side, instead of having a head and tail, and back and belly.
  • As adults they have a internal skeleton formed by thick calcified plates that fuse inside the entire body.
  • The development of sea urchins is influenced by thyroid hormones in larvae that help them grow into their adult bodies. The larvae probably feed on algae which is plentiful in these hormones, since inhibitors of TH production do not lead to lower levels. TH levels peak right before they are about to transform into full adults. (14) (DM)
A picture outlining the process of early development. (3) (E.S.S.)


  • Sea Urchins have thin layers of skin on the outside of their bodies.
  • Sea urchins have an outer shell, called the test, in which “spines” or the spikes protrude. The outer shell and the spines protect them from predators.(LC) 2
  • Sea urchin spines posses an indented base which fits like a ball-and-socket joint over the urchin’s spherical shell. Muscle fibers wrap around the shell and facilitate movement. Most spines are solid with a blunt tip, but in some species, they are hollow with sharp tips. They contain a venom which can include a neurotoxin and/or a cardiovascular toxin. [4] (CC)


  • Because of the radial symmetry, Sea Urchins are able to move equally as well in all directions, however not quickly.
  • Larva move by beating the cilia, little extensions, that cover its skin.
  • Adults have extensions called tube feet which help the Sea Urchin move around on the sea floor.
  • Each spine has neural innervation. As a spine is stimulated, neighboring spines bend in concert and help facilitate movement. [5] (CC) (JLau)

Sensing the Environment

  • "Sea urchins, like their close relatives the sea stars (starfish), don't technically have eyes. Instead, the ball-like invertebrates detect light striking their spines and compare the beams intensities to get a sense of their surroundings. Even though urchins don't have eyes, their visual abilities are similar to those of marine invertebrates that do have eyes, such as the nautilus and the horseshoe crab." (8) (MC)

  • Sea urchins coordinate their balance and remain upright by their statocysts, balance sensory receptors which indicate to the organism the current position of the organism. The statocyst is shaped like a hollow ball. The inside edges of the ball is full of setae, sensory hairs. Inside the ball is a small mass called a statolith. Gravity acts on the statolith and causes it to roll around inside the statocyst, stimulating different setae as the statolith rolls over them. The specific setae activated tells the urchin which position it is in, whether it is rolled on its side, sitting upright, or upside down. [11][12](AY)

Gas Exchange

  • The water vascular system which is a network of water filled canals plays a role in gas exchange
  • Gas exchange occurs mainly on the thin skin of the tube feet that covers the sea urchin and, to a lesser extent, on the gills on the oral surface. The sea urchin does not have a circulatory system like mammals do, but rather the gases move to and from organs via the coelomic fluid which fills the main body cavity. (5) (AA)

Waste Removal

  • Sea Urchins have an aboral side which contains an anus which excretes waste from the body.

Environmental Physiology

  • Because Sea Urchins live in salt water, they need a pH level of 6.0-9.0. Sea Urchins are accustomed to the tropical coral reefs and therefore need a water temperature around 82 degrees Fahrenheit to be most comfortable. They live in salt water but they are highly sensitive to the salt concentration. (16)(ES)

Internal Circulation

  • Using its vascular water system the Sea Urchin is able to circulate water, nutrients and waste.
  • It also has a hemal system where the blood is located. The fluid(water and blood) circulates the entire body and has coelomocytes or immune cells that do phagocytosis. Coelomocytes help in the clotting of blood while also getting rid of waste in the body via the gills and tube feet. (9)(NC)

Chemical Control

Fun Fact of the Day!
The name "urchin" is an old name for a type of round, spiny hedgehogs that the sea urchins resemble! (Shwetha)(9)

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Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price.
Principles of Life High School Edition.
Sudnerland, MA: Sinauer Associates, 2012. Print.2.
10.Busdraghi, Marco. N.d. Photograph. N.p. found<>

Review Questions1. What is the name of the phenomenon that allows a dislodged piece of a sea urchin to develop into an entirely new sea urchin? (JLev)2. What is the structure and the function of the acrosomal process? (BB-V)3. How do urchins feed themselves? How do their anatomy aid them in doing so? (AWC)